How To Render A Blob On A Canvas Element?

How to render an image blob to a canvas element? So far i have these two (simplified) functions to capture an image, transform it to a blob and eventually render the blob on a canv

Solution 1:

I think you need to tidy up your code a bit. It's hard to know what you are trying to achieve because there are many unnecessary lines of code. The main problem is that blob is coming undefined here

HTMLCanvasElement.prototype.renderImage = function(blob){

because photo never gets initialized here inside the toBlob function...which is unnecessary for what you are trying to achieve.

Here's a simplified working version of your code snippet

var canvas = document.getElementById('canvas');
var input = document.getElementById('input');


  functionpicToBlob() {
    canvas.renderImage(input.files[0]);
  }

HTMLCanvasElement.prototype.renderImage = function(blob){
  
  var ctx = this.getContext('2d');
  var img = newImage();

  img.onload = function(){
    ctx.drawImage(img, 0, 0)
  }

  img.src = URL.createObjectURL(blob);
};

input.addEventListener('change', picToBlob, false);

<inputtype='file'accept='image'capture='camera'id='input'><canvasid = 'canvas'></canvas>

Solution 2:

You can use it as below

functionrenderImage(canvas, blob) {
  const ctx = canvas.getContext('2d')
  const img = newImage()
  img.onload = (event) => {
    URL.revokeObjectURL(event.target.src) // 👈 This is important. If you are not using the blob, you should release it if you don't want to reuse it. It's good for memory.
    ctx.drawImage(event.target, 0, 0)
  }
  img.src = URL.createObjectURL(blob)
}

---

below is an example

/**
 * @param {HTMLCanvasElement} canvas: https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API
 * @param {Blob} blob: https://developer.mozilla.org/en-US/docs/Web/API/Blob
 * */functionrenderImage(canvas, blob) {
  const ctx = canvas.getContext('2d')
  switch (blob.type) {
    case"image/jpeg": // Normally, you don't need it (switch), but if you have a special case, then you can consider it.case"image/png":
      const img = newImage()
      img.onload = (event) => {
        URL.revokeObjectURL(event.target.src) // Once it loaded the resource, then you can free it at the beginning.
        ctx.drawImage(event.target, 0, 0)
      }
      img.src = URL.createObjectURL(blob)
      break  
  }
}

// 👇 below is test
(() => {
  const canvas = document.querySelector('canvas')
  const input = document.querySelector('input')
  input.addEventListener('change',
    (event) => {
      const file = event.target.files[0]
      const blob = newBlob(
        [file],
        {"type": file.type} // If the type is unknown, default is empty string.
      )
      renderImage(canvas, blob)
    }
  )
})()

<div><inputtype='file'accept='.png,.jpg'></div><canvas></canvas>

another example to show you What effect of the revokeObjectURL.

<div></div><canvaswidth="477"height="600"></canvas><script>asyncfunctionrenderImage(canvas, blob, isNeedRevoke=true) {
    const ctx = canvas.getContext('2d')
    const img = newImage() // The upper part of the painting.const img2 = newImage() // The lower part of the painting.awaitnewPromise(resolve => {
      img.onload = (event) => {
        if (isNeedRevoke) {
          URL.revokeObjectURL(event.target.src)
        }
        ctx.drawImage(event.target,
          0, 0, 477, 300,
          0, 0, 477, 300
        )
        resolve()
      }
      img.src = URL.createObjectURL(blob)
      setTimeout(resolve, 2000)
    }).then(() => {
      img2.onload = (event) => {
        ctx.drawImage(event.target,
          0, 300, 477, 300,
          0, 300, 477, 300
        )
      }
      img2.src = img.src// 👈 If URL.revokeObjectURL(img.src) happened, then img2.src can't find the resource, such that img2.onload will not happen.
    })
  }

  functionCreateTestButton(canvas, btnText, isNeedRevoke) {
    const button = document.createElement("button")
    button.innerText = btnText
    button.onclick = async (event) => {
      canvas.getContext("2d").clearRect(0, 0, canvas.width, canvas.height)  // clear canvasfetch("https://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/477px-PNG_Test.png")
        .then(async response=>{
          const blob = await response.blob()
          renderImage(canvas, blob, isNeedRevoke)
        }).catch(err=>console.error(err))
    }
    return button
  }

  (() => {
    window.onload = () => {
      const canvas = document.querySelector('canvas')
      const div = document.querySelector('div')
      const btn1 = CreateTestButton(canvas, "Without URL.revokeObjectURL", false)
      const btn2 = CreateTestButton(canvas, "URL.revokeObjectURL", true)
      div.append(btn1, btn2)
    }
  })()
</script>